Alexander disease is an uncommon but fatal central nervous system (CNS) disorder. Alexander disease usually affects children. In the most common form, Alexander disease begins its manifestations before the age of 2 years as megalencephaly and white matter loss, especially in the frontal lobes. Seizures and spasticity that are difficult to control are prominent symptoms, as are hydrocephalus and psychomotor developmental delay. The prognosis for individuals with Alexander disease is generally poor. Most children with the infantile form do not survive past the age of 6. In the juvenile form, death usually occurs within 10 years after the onset of symptoms.
The key diagnostic feature of the neuropathology in Alexander disease is widespread deposition of Rosenthal fibers in subpial, periventricular, and white matter astrocytes throughout the CNS. Morphologically, Rosenthal fibers consist of two components, bundles of intermediate filaments and surrounding irregular deposits of dense material. Biochemically, Rosenthal fibers are composed of a complex ubiquitinated mixture of the intermediate filament glial fibrillary acidic protein (GFAP) in association with other constituents, especially the small stress proteins αB-crystallin and HSP25. Rosenthal fibers are also found sporadically in several other settings, such as some astrocytomas and chronic gliosis; their original description was in a young adult with syringomyelia, but never to the extent seen in Alexander disease.
GFAP is the major structural protein in astrocytes, and the initiation of GFAP synthesis as astrocytes mature has traditionally been considered a key step in their differentiation. Studies in cultured astrocytes and cell lines of forced over-expression of GFAP or its antisense inhibition suggest that GFAP directly controls process outgrowth. Recent studies have also documented expression of GFAP at low levels in a multipotential stem cell that gives rise to both neurons and glia, as do later-forming radial glia. The transcriptional regulation of GFAP has been reviewed, and there are also several minor isoforms about which much less is known than the major GFAPa species of mRNA. One such isoform, termed GFAPE, arises by alternative splicing and produces a protein that interacts with presenilins in the brain, although the precise cell in which GFAPE is expressed is not yet clear. Other GFAP isoforms are expressed at low levels in several cells outside of the central nervous system, such as non-myelinating Schwann cells, enteric glia, lens epithelium, and hepatic stellate cells, but at much lower levels than the GFAPα in mature astrocytes. Given the breadth of cell types that express some form of GFAP, the consequences of GFAP-deficiency or deletion cannot easily be predicted.
In 1995-1996, generation of GFAP-null mice was reported by four different research groups. The consistent result was the surprising finding that GFAP-null mice are viable, with seemingly normal life spans, reproduction, and gross motor behavior. Ultrastructural studies in optic nerve and spinal cord suggested minor changes in astrocyte morphology, with shortening and thinning of astrocyte processes; this was supported with subsequent studies of dye-filled cells. With regard to neuronal function, subtle changes were identified in long-term depression in the cerebellum, and long-term potentiation in the hippocampus. There is some evidence for alterations in the blood brain barrier derived from cell culture models, and to a limited extent from studies in vivo. No defects have been found in any of the non-astrocytic cells that express minor GFAP isoforms.
The GFAP gene has been evaluated in at least 130 Alexander disease patients. Mutations are present in nearly all (˜95%) cases of infantile Alexander disease, and at least a certain proportion of juvenile and adult cases. Remarkably, mutations at only two amino acids, Arg79 or Arg239, account for nearly half of all cases. All of the mutations are heterozygous, presumably acting in an autosomal dominant fashion. For all of the cases where parents were available for testing, the parents were normal (i.e. did not have the mutation present in their child), confirming that the mutations occurred de novo. Examples of GFAP mutations being inherited occurred in families of adult-onset cases, one being where a mother and two adult children were affected and carried the same mutation. The penetrance also approaches 100%, two exceptions being two children whose initial evaluation for other problems led to MRI diagnoses of leukodystrophy, with subsequent genetic analysis revealing GFAP mutations, and one of these children is now showing signs of Alexander Disease.
Many of the GFAP mutations occur within an amino acid sequence that is highly conserved among intermediate filament proteins. Mutations at the homologous sites of other intermediate filament proteins are also associated with human diseases involving blistering of the skin, cataracts, cardiomyopathies, and muscular dystrophies. Although homologous sites are affected, most of these other mutations lead to a dominant loss of function. GFAP mutations, on the other hand, appear to produce a dominant gain of function. For example, a loss of function is indicated for keratin mutations because both heterozygous mutations in humans and null mutations of the homologous gene in mice disrupt the keratin filament network and produce a similar blistering disease. In contrast, GFAP filaments are present in Alexander disease patients, and GFAP null mice are fully viable and their pathology does not resemble Alexander disease. Thus the GFAP mutations do not appear to act by reducing or eliminating normal GFAP function, but rather by producing a new, deleterious, activity.
Two studies have attempted to link GFAP deficiency with abnormalities of myelination. One of these reported that approximately half of the GFAP-null mice in their colony developed an adult onset degeneration of myelin, with resulting hydrocephalus. Grossly visible changes in cerebral white matter did not become apparent until mice were 18 months and older. The mechanism of this myelin degeneration has not been determined, and the possible contribution of background genetics (for instance, corpus callosum defects in 129 strain mice) has not been excluded. Furthermore, this finding has not been confirmed by any of the other groups working on GFAP-null mice. The second report determined that GFAP-null mice had increased susceptibility to experimental allergic encephalitis, with increased clinical scores and pathology. However, inflammation is thought to play little role in the pathogenesis of Alexander disease.
Overall, the relevance of GFAP-null mice to Alexander disease remains uncertain. Such mice do not develop a severe leukodystrophy and have no Rosenthal fibers, two primary characteristics of the human disease. In addition, the GFAP-null mice do not exhibit developmental delay, spasticity, seizures, ataxia, or paralysis. It remains formally possible that some aspects of the Alexander phenotype reflect dominant negative effects, and studies with keratin 10 have shown that dominant negatives may sometimes produce more severe phenotypes than corresponding null mutations. However, for GFAP, most evidence suggests that a toxic gain of function may be implicated as the mechanism of action.